Chronic infection with HCV is a major health problem associated with liver cirrhosis, hepatocellular carcinoma and liver failure. An estimated 170 million chronic carriers worldwide are at risk of developing liver disease. See, for example, Szabo, et al., Pathol. Oncol. Res. 2003, 9:215-221, and Hoofnagle J H, Hepatology 1997, 26:15 S-20S. In the United States alone 2.7 million are chronically infected with HCV, and the number of HCV-related deaths in 2000 was estimated between 8,000 and 10,000, a number that is expected to increase significantly over the next years. Infection by HCV is insidious in a high proportion of chronically infected (and infectious) carriers who may not experience clinical symptoms for many years. Liver cirrhosis can ultimately lead to liver failure. Liver failure resulting from chronic HCV infection is now recognized as a leading cause of liver transplantation.
HCV is a member of the Flaviviridae family of RNA viruses that affect animals and humans. The genome is a single ˜9.6-kilobase strand of RNA, and consists of one open reading frame that encodes for a polyprotein of ˜3000 amino acids flanked by untranslated regions at both 5′ and 3′ ends (5′- and 3′-UTR). The polyprotein serves as the precursor to at least 10 separate viral proteins critical for replication and assembly of progeny viral particles. The organization of structural and non-structural proteins in the HCV polyprotein is as follows: C-E1-E2-p7-N52-N53-NS4a-NS4b-NS5a-NS5b. Because the replicative cycle of HCV does not involve any DNA intermediate and the virus is not integrated into the host genome, HCV infection can theoretically be cured. While the pathology of HCV infection affects mainly the liver, the virus is found in other cell types in the body including peripheral blood lymphocytes. See, for example, Thomson B J and Finch R G, Clin Microbial Infect. 2005, 11:86-94, and Moriishi K and Matsuura Y, Antivir. Chem. Chemother. 2003, 14:285-297.
At present, the standard treatment for chronic HCV is interferon alpha (IFN-alpha) in combination with ribavirin and this requires at least six (6) months of treatment. IFN-alpha belongs to a family of naturally occurring small proteins with characteristic biological effects such as antiviral, immunoregulatory and antitumoral activities that are produced and secreted by most animal nucleated cells in response to several diseases, in particular viral infections. IFN-alpha is an important regulator of growth and differentiation affecting cellular communication and immunological control. Treatment of HCV with interferon has frequently been associated with adverse side effects such as fatigue, fever, chills, headache, myalgias, arthralgias, mild alopecia, psychiatric effects and associated disorders, autoimmune phenomena and associated disorders and thyroid dysfunction. Ribavirin, an inhibitor of inosine 5′-monophosphate dehydrogenase (IMPDH), enhances the efficacy of IFN-alpha in the treatment of HCV. Despite the introduction of ribavirin, more than 50% of the patients do not eliminate the virus with the current standard therapy of interferon-alpha (IFN) and ribavirin. By now, standard therapy of chronic hepatitis C has been changed to the combination of pegylated IFN-alpha plus ribavirin. However, a number of patients still have significant side effects, primarily related to ribavirin. Ribavirin causes significant hemolysis in 10-20% of patients treated at currently recommended doses, and the drug is both teratogenic and embryotoxic. Even with recent improvements, a substantial fraction of patients do not respond with a sustained reduction in viral load and there is a clear need for more effective antiviral therapy of HCV infection. See, for example, Fried, et al. N. Engl. J Med 2002, 347:975-982.
A number of approaches are being pursued to combat the virus. They include, for example, application of antisense oligonucleotides or ribozymes for inhibiting HCV replication. Furthermore, low-molecular weight compounds that directly inhibit HCV proteins and interfere with viral replication are considered as attractive strategies to control HCV infection. Among the viral targets, the NS3/4A protease/helicase and the NS5b RNA-dependent RNA polymerase are considered the most promising viral targets for new drugs. See, for example, Ni, Z. J. and Wagman, A. S. Curr. Opin. Drug Discov. Devel. 2004, 7, 446-459, Beaulieu, P. L. and Tsantrizos, Y. S. Curr. Opin. Investig. Drugs 2004, 5, 838-850, and Griffith, et al., Ann. Rep. Med. Chem 39, 223-237, 2004.
Besides targeting viral genes and their transcription and translation products, antiviral activity can also be achieved by targeting host cell proteins that are necessary for viral replication. For example, Watashi, et al, Molecular Cell, 19, 111-122, 2005, show how antiviral activity can be achieved by inhibiting host cell cyclophilins. Alternatively, a potent TLR7 agonist has been shown to reduce HCV plasma levels in humans. See, Horsmans, et al, Hepatology, 42, 724-731, 2005.
Compounds said to be useful for treating HCV infection are disclosed, for example, in WO 2008/064218 (Leivers et. al), WO 2008/244380 (Bachand et. al), and US 2009/0068140 (Bachand et. al). These references also disclose methods for preparing the compounds, compositions comprising the compounds, pharmaceutical compositions comprising the compounds and additional compounds, methods of treating HCV, salts of the compounds, routes of administration, and other information regarding how to make, formulate, and use the compounds.